Level measuring devices which operate according to the frequency modulated continuous wave (FMCW) principle should have relatively great dynamics, since all the echoes are at the receiver at the same time. The various echoes can be, for example, small and relatively far-off useful echoes as well as relatively strong spurious echoes from the short range of the antenna. These spurious echoes can be caused, for example, by deposit build-up in or on the antenna, by fixtures in front of the antenna or by a finite uncoupling of the duplexer in the HF module of the level radar.
Furthermore, echoes in the short range of the antenna can generally have very high amplitudes in contrast to echoes which are further away and have extremely low amplitudes.
To be able to process all these echoes in an intermediate frequency channel (IF channel), then in this case as well, the IF dynamics of, for example, the analog/digital converter provided in the receiving branch of the level measuring device may have to be very high.
A widely-used option of compressing the dynamics, i.e. of reducing the dynamic range, is to use a so-called STC function. In the present context, “STC” stands for “Sensitivity Time Control”.
The STC function is used to attenuate the large signals in the short range of the antenna and can be realised very easily in an FMCW system by a high-pass filter.
A high-pass filter can attenuate the signals with relatively small frequencies. Since the small frequencies correspond to a short distance from the antenna, the disturbing, large signals in the short range of the antenna can be attenuated or can even be completely filtered out as a result, thereby reducing the differences in the amplitudes of the IF signal.
However, the course of the curve of the measuring signal can also be influenced by the distance-dependent attenuation of the echo signals which, in turn, can affect the accuracy of the sensor in the short range.